Tomato is a basic nutritional component in many cultures worldwide. It is known for its vitamin, mineral and anti-oxidant content that provide the consumers with these health beneficial components. A large number of tomato varieties is available in the market, and it is recognized that in addition to the nutritional and taste parameters, color and general appearance influence the purchase of tomato fruit by private as well as cuisine professional customers.
Tomato fruit color is furnished by lycopene, a linear carotene molecule with 11 conjugated double bonds, which accumulates to high concentration within chromoplasts during fruit ripening. Biosynthesis of lycopene is mainly controlled at the gene expression level of carotenoid-biosynthesis enzymes. During fruit ripening, genes for enzymes upstream to lycopene are upregulated whereas genes for lycopene cyclases, which metabolize lycopene to alpha- and beta-carotene, are silenced.
The improvement of crop species has been a fundamental human pursuit since the beginning of agriculture. One can imagine that unique plant phenotypes, sometimes represented even by a single plant, caught the eyes of the first ancient breeders. The manner in which the improvement process was done constituted a founder effect which is often manifested in severe genetic bottlenecks. As a result of these genetic bottlenecks imposed during early domestication and modern breeding activities, cultivated varieties of most crop species carry only a small fraction of the variation presented in their wild ancestors and land races (Tanksley S D and McCouch S R. 1997. Science 277:1063-1066). The narrowed genetic variation in modern crops is one of the causes for genetic vulnerability to various plant diseases and pests. Moreover, it presents a breeding obstacle by lowering the chance of obtaining better gene and allelic combinations to improve traits with agricultural importance.
One of the most powerful and efficient method in the area of genetics, molecular biology and plant breeding is based on mutant variation. Mutation is the major evolutionary force that creates variation to improve survivability of existing diversity and for the evolution of novel ecotypes, races and species. However, since the frequency of spontaneous mutants is very low, they supply information on a relatively small number of genes and biological phenomena. Therefore, artificial mutagenesis methods have been developed and are being applied to induce variation. Induced variation is in turn used as a tool for the discovery of gene function and for understanding developmental processes.
An available source of tomato mutations is an isogenic tomato “mutation library” generated in the genetic background of the processing tomato inbred variety M82, by one of the inventors of the present invention and co-workers (Menda N et al., 2004. Plant J. 38:861-872). For generating the library, a total of 13,000 M2 families, derived from ethyl methanesulfonate (EMS) chemical treatment and from fast-neutron mutagenesis of seeds, were phenotyped when grown under field conditions. Based on the phenotypes, the families were categorized into a morphological catalog that included 15 primary and 48 secondary categories. More than 3000 mutations have been identified in this library. Some of the mutations represent new alleles of previously described phenotypes from the monogenic mutant collection of The Tomato Genetics Resource Center (TGRC), most of which in the M82 isogenic population. In addition, over 1,000 novel phenotypes with multiple alleles per locus were identified.
Results of allelism tests between mutants sharing similar phenotypes verified the hypothesis that the population is reaching saturation, with hits in the majority of the genes (for example, from the dominant LANCEOLATE mutation 8 independent alleles where identified; from the yellow fruited Yellow flesh mutation 5 alleles were obtained). In addition, screening of 1,000 EMS families subjected to two treatments of EMS did not result in novel phenotypes.
The potential of wild species as a source of genetic variation to bring about crop improvement was recognized early in the twenty first century (Zamir D. 2001. Nat Rev Genet. 2:983-989). Initial interspecific breeding attempts met with severe problems, including incompatibility in crosses between the wild species and the cultivated crops; F1-hybrid sterility; infertility of the segregating generations; reduced recombination between the chromosomes of the wild species and the cultivated crop; and tight linkage between genes that have negative effects and the traits of interest (McCouch S. 2004. PLoS Biol. 2). Despite these obstacles, there are many examples in which wild introgression breeding has made a considerable contribution to the development of modern-day varieties, predominantly as a source for monogenic or sometimes called single gene traits and to a smaller extent for complex traits such as yield, compositional quality and resistance to various stresses that are influenced by quantitative trait loci (QTL; Fernie A R et al., 2006. Curr. Opin. Plant Biol. 9:196-202).
Notwithstanding the above-described tools for generating new tomato phenotypes, there is a constant market demand for stable cultivars having fruit with new, attractive color.